The present invention relates to a fuel injection system for an internal combustion engine in which liquid fuel under high pressure is fed into a fuel injection valve or injector and charged therefrom into the engine by reciprocation of a plunger.
In an internal combustion engine, particularly a diesel engine, a fuel injection system has customarily been furnished with a relatively long delivery or injection tubing for hydraulic communication between a fuel injection pump and a fuel injection valve or injector. Fuel is pressurized by a plunger included in the pump and then fed to the fuel injector via the long injection tubing to be charged into the engine therefrom. With this system, the length, volume and other factors of the injection tubing have substantial influence on the supply of fuel under pressure by the plunger which makes it impossible to fully control the rate of injection at the injector through a control of the delivery rate at the plunger. For instance, even if a cam associated with the plunger moves the same angular distance driving the plunger to deliver fuel for the same time period, the injector may actually inject fuel for a time period which grows longer in accordance with the revolution speed of the engine. A difficulty is also experienced in controlling the quantity of injection in a particular range wherein the injection quantity is relatively small.
To settle these problems attributable to the long injection tubing, various types of unit injectors have hitherto been proposed which are commonly designed to omit an injection tubing by having an integral arrangement of a fuel injection pump and a fuel injection valve. A typical example of such unit injectors is disclosed in U.S. Pat. No. 4,069,800 assigned to the same assignee as the present application in which a servo piston is driven to compress fuel in a pressure chamber which communicates with a needle valve which is spring biased to block communication between the pressure chamber and a fuel injection nozzle. The servo piston which is of an automatic valve type is disposed in the injector body to be operated by hydraulic fluid pressure through a solenoid operated directional control valve or selector. High pressure fuel is supplied by the servo piston to an injector which is positioned at the end of the injector body, thus being charged into the engine at a high pressure. Such a unit injector succeeds in pressurizing low pressure fuel within the injector body up to a level high enough for the injection into the engine.
However, a unit injector of the type described cannot avoid various and critical problems originating from its inherent construction and arrangement. Provision of a servo piston in the injector body offers an intricacy of construction. A disproportionate mechanical strength is required for various portions of the injector body, particularly the seat of the valve needle of the injector, the stand the high fuel pressure. Meanwhile, the injection pressure of fuel on which the control of injection rate depends and injection timing, particularly time period or duration of direct injection, need be adjusted accurately in accordance with various demands of an engine to cut down fuel consumption, reduce engine noise and promote efficient exhaust emission control. The unit injector fails to meet this requirement for three different reasons: that the servo piston provides only a predetermined pressure of injection, that the injection starting timing is dependent solely on the pressure at which the needle valve will be opened by the high pressure fuel, and that a force necessary for closing the needle valve is so large that it is quite difficult to control the injection terminating timing. Additionally, the unit injector is unsuitable for use in a high speed engine because the supply of the working hydraulic fluid through the directional control valve limits the responsiveness of the servo piston.